Merge pull request #3423 from matijazigic:fix_optflow_rlof

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- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake


The bug has already been reported and explained on the OpenCV forum:

- [optflow-calcopticalflowsparserlof-giving-assertion-error](https://forum.opencv.org/t/cv2-optflow-calcopticalflowsparserlof-giving-assertion-error/4835)
- [calcopticalflowsparserlof-assertion-error-in-initialflow](https://forum.opencv.org/t/calcopticalflowsparserlof-assertion-error-in-initialflow/8669)

The bug is manifested only if 'UseInitialFlow' is set to true and 'ForwardBackwardThreshold' > 0 (enabled).

Author: matijazigic

modules/optflow/src/rlofflow.cpp

@@ -408,8 +408,13 @@ class SparseRLOFOpticalFlowImpl : public SparseRLOFOpticalFlow
         cv::Mat(1,npoints , CV_32FC2, &nextPoints[0]).copyTo(nextPtsMat);
         if (forwardBackwardThreshold > 0)
         {
+            // use temp variable to properly initialize refPoints
+            // inside 'calcLocalOpticalFlow' when 'use_init_flow' and 'fwd_bwd_thresh' parameters are used
+            bool temp_param = param->getUseInitialFlow();
+            param->setUseInitialFlow(false);
             // reuse image pyramids
             calcLocalOpticalFlow(nextImage, prevImage, currPyramid, prevPyramid, nextPoints, refPoints, *(param.get()));
+            param->setUseInitialFlow(temp_param);
         }
         for (unsigned int r = 0; r < refPoints.size(); r++)
         {